A mobile communications system generally refers to any telecommunications system wherein the access point (typically wireless access) to the system can change when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with a wireless access to external networks, hosts, or services offered by specific service providers.
Currently, third-generation mobile communication systems, such as the Universal Mobile Communication System (UMTS) and the Future Public Land Mobile Telecommunication System (FPLMTS), which was later renamed as the International Mobile Telecommunication 2000 (IMT-2000) are under development. The UMTS is being standardized by the European Telecommunication Standards Institute (ETSI), whereas the International Telecommunication Union (ITU) standardizes the IMT-2000 system. These future systems are basically very similar. For example the UMTS, as all mobile communication systems, provides wireless data transmission services to mobile subscribers. The system supports roaming, which means that UMTS users can be reached and they can make calls anywhere as long as they are situated within the coverage area of the UMTS.
In the UMTS architecture the UMTS terrestrial radio access network, UTRAN, consists of a set of radio access networks RAN (also called radio network subsystem RNS) connected to a core network CN through the interface Iu. Each RAN is responsible for the resources of its set of cells. For each connection between a mobile station MS and the UTRAN, one RAN is a serving RAN. A RAN consists of a radio network controller RNC and a plurality of base stations BS. The RNC is responsible for the handover decisions that require signalling to the MS. The base stations are connected to the RNC through the Iub interface. The base station BS communicates with the mobile stations MS (or user equipments UE) over the radio interface Uu. The Uu and Iub interfaces are not relevant to the present invention and will not be described in more detail herein. Further information can be found in the UMTS specifications.
In the interface Iu between the radio network controller RNC and the core network the transfer technique is the ATM (Asynchronous Transfer Mode). The ATM transmission technique is a switching and multiplexing solution particularly relating to a data link layer (i.e. OSI layer 2, hereinafter referred to as an ATM layer. In the ATM data transmission the end users data traffic is carried from a source to a destination through virtual connections. Data is transferred over switches of the network in standard-size packets called ATM cells. The ATM cell comprises a header, the main object of which is to identify a connection number for a sequence of cells forming a virtual channel (VC) for a particular call through the transport network. A physical layer (i.e. OSI layer 1) may comprise several virtual paths (VP) multiplexed in the ATM layer. Each virtual path includes several VCs.
One core network which will use the UMTS radio access network is the general packet radio service (GPRS) which is a new service for the GSM system (Global System for Mobile communication), and a similar service is also being defined for the 3G mobile systems. A subnetwork comprises a number of packet data service nodes SN, which in this application will be referred to as serving GPRS support nodes SGSN (or 3G-SGSNs in the 3G systems). As illustrated in FIG. 1, each 3G-SGSN is connected to the RNC in the UTRAN over a transport network so that the 3G-SGSN can provide a packet service for mobile data terminals via several base stations, i.e. cells. The intermediate UTRAN provides a radio access and a packet-switched data transmission between the 3G-SGSN and mobile stations MS. Different subnetworks are, in turn, connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. The GPRS service thus allows to provide packet data transmission between mobile data terminals and external data networks when the UTRAN (or the GSM) network functions as a radio access network.
In order to guarantee interoperability between different vendors of networks and network elements, the present 3G specifications specify that the RNC and the 3G-SGSN are connected using point-to-point ATM permanent virtual channel (PVC) connections. These signals can be carried over different transport networks, such as the ATM network or the Synchronous Digital Hierarchy (SDH) network. The 3G specifications do not, however, specify how these ATM PVC connections are set up but allow the operators and vendors to use different solutions.
A problem in such a system may be fault tolerance of the ATM PVC connections. In the worst case, a failure in the PVC connections or in the interfaces at the RNC and the 3G SGSN may block all communication. The current 3GPP Iu interface specifications only specify that if redundancy of pathways of the ATM layer between the CN and the RNC is supported, it shall be implemented using ATM protection switching according to ITU-T recommendation I.630. Since I.630 does not support 1:n and m:n architectures another backup PVC connection is required for each ATM PVC connection, which doubles the number of ATM PVCs between RNC and 3G-SGSN.
Most present ATM switches do not support ATM layer protection according I.630. This means that the use of the ATM protection between ATM edge switch and RNC/3G-SGSN is not necessarily possible. Also the use of end-to-end ATM layer protection between the RNC and the 3GSGSN is not possible, if the ATM network does not support ATM OAM according I.610 (i.e. does not generate end-to-end ATM-AIS cells in case of link failure).